Method and apparatus for determining path attenuation of radio waves

ABSTRACT

A method and apparatus for determining path attenuation of radio waves in a radio system, in which at least a two-dimensional vector map describing the environment of a base station is used for determining the coverage area of the base station of the system, and in which the strength of the emission of a transmitter is determined at various points in the environment. The invention is implemented by calculating from the base stations environment described by the vector map a polygon area to which radio waves can propagate both directly and by means of diffraction and reflections, by estimating the strength of the emission of the transmitter at each location point of the transmitter to be examined in the polygon area determined by calculation, and by determining points of the polygon area in advance, storing the determined points in a memory, and reading the stored points from the memory for calculating the polygon area for different location points of the transmitter.

FIELD OF THE INVENTION

The present invention relates to a method for determining pathattenuation of radio waves in a radio system, in which method at least atwo-dimensional vector map describing the environment of a base stationis used for determining the coverage area of the base station of thesystem, and in which the strength of the emission of a transmitter isdetermined at various points in the environment.

The invention further relates to an apparatus for determining pathattenuation of radio waves in a radio system comprising at least onebase station, and which apparatus comprises means for maintaining atleast a two-dimensional vector map describing the environment of adesired base station, and means for determining the strength of theemission of a transmitter at various points in the environment.

BACKGROUND OF THE INVENTION

When building up a radio system, the desired coverage area is to beattained with as low costs as possible. When the locations of the basestations of the system are considered, both the required trafficcapacity and the coverage area to be attained are taken into account.The base stations tend to be placed so that the coverage area to beattained is great and the base station is situated in a preferablelocation for the propagation of radio waves. For this purpose, there arevarious methods and auxiliary means in radio network planning. Vectormaps to which the terrain and building information of a desired area ismapped are generally used as auxiliary means in radio network planning.By means of a vector map, with the help of a computer, coverage areasand parameters associated with the operation of the network can becalculated for various locations of base stations.

The present invention can be applied to cellular radio networksespecially in an environment having many buildings where the radionetwork is realized by means of microcells. Microcelis refer to cellsconsiderably smaller in size than conventional cells in the cellularradio system. Microcells are typically used in the city centres where alot of traffic capacity is needed.

When path attenuation of a radio wave is calculated in the micro cellenvironment, the propagation mechanisms are generally divided intodiffraction, reflections, scattering and direct wave. When this kind ofa deterministic propagation model is used in radio network planning, theproblem will be the length of time spent for calculation.

For example, multiple diffraction is calculated slowly because therewill be very many different combinations of connections between cornerseven in simple building maps. The method of double diffraction isgenerally used in the propagation model calculation where the cornersvisible to the transmitter are examined. It is tested at each cornerwhich calculation points are in the line of vision of the corner. Thediffraction field of this corner is calculated therein. It is furthertested which corners are in the line of vision of this corner and thesecorners and calculation points in the line of vision of the corners areexamined. The method is very slow as it has to be calculated at eachcalculation point if there is visibility to the corner to be diffracted.If there are many calculation points, in grid maps generally (10⁶ -10⁷),the visibility check of each point to every corner to be diffracted is avery laborious operation.

A corresponding situation also occurs when calculating reflections. Aso-called ray-tracing method is used for calculating multiplereflections. Ray-tracing can be carried out in two ways: by means ofmultiple mirror images or by means of a ray launching method.

The problem with determining multiple reflection routes by means ofmirror images is that the number of different wall combinations andmirror images is generally very high and the required calculation isarduous and slow. Similarly, the calculation capacity required for theray launching method is great and the method is also inaccurate.

CHARACTERISTICS OF THE INVENTION

The object of the present invention is to accomplish a method and anapparatus by means of which the disadvantages of prior art methods canbe obviated. The object of the invention is to achieve a method forcalculating the coverage area of the base station in such a manner thatthe required amount of calculation is considerably smaller than in priorart methods. The purpose of the invention is to provide a method forcalculating the coverage area of a base station fast and advantageously.

This will be attained with the method as shown in the preamble which ischaracterized by calculating from the calculation area described by thevector map a polygon area to which radio waves can propagate bothdirectly and by means of diffraction and reflections, and by estimatingthe strength of the emission of the transmitter at each location pointof the transmitter to be examined in the polygon area determined bycalculation.

The apparatus of the invention is characterized in that the apparatusfurther comprises means for calculating from the calculation areadescribed by the vector map a polygon area to which radio waves canpropagate both directly and by means of diffraction and reflections, andmeans for estimating the strength of the emission of the transmitter ateach location point of the transmitter to be examined inside thedetermined polygon area.

The solution of the invention has many advantages. The great amount ofcalculation required by known methods can be diminished by restrictingthe area of the vector map only to those areas on which the radio wavescan propagate. The method makes calculation many times faster. Theacceleration of calculation is very significant for radio networkplanning where the received power is calculated on the vector map whichcan easily be transferred to a computer display, for example. In networkplanning, the exact location of the base station and the features of thesystem are selected on the basis of-the calculated result. Networkplanning is often an iterative process where the position of the basestation is changed many times to achieve the desired result. By means ofexact and fast calculation, the costs of network planning can bedecreased and the efficiency of planning increased.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the invention will be explained in more detail bymeans of the appended drawings, wherein

FIG. 1 shows an example of a radio system to the planning of which themethod of the invention can be applied,

FIG. 2 illustrates an exemplary part of a vector map of a town area,

FIGS. 3a and 3b illustrate an example of calculating diffractions,

FIG. 4 illustrates an example of calculating reflections by the mirrorimage principle,

FIG. 5 illustrates an example of the solution of the invention forcalculating a reflection polygon, and

FIG. 6 illustrates the structure of the apparatus of the invention onthe block diagram level.

BRIEF DESCRIPTION OF THE INVENTION

The method of the invention can hence be applied preferably to radionetwork planning. It is especially suitable for planning radio systemswhich are accomplished with microcell technique in a town environment.The structure of one typical radio system is described by way of examplewith relevant parts in FIG. 1. The system comprises a set of basestations 100, 116 and a set of generally mobile subscriber terminalequipments 102 to 106 which have a bidirectional connection 108 to 112to some base station 100, 116. The base stations 100, 114 transmit theconnections of the terminal equipments 102 to 106 to a base stationcontroller 114 that forwards them to other parts of the system and tothe fixed network. The base station controller 114 controls theoperation of one or more base stations 100, 116. In network planning,the base stations 100, 116 are placed as preferably as possible in sucha way that the coverage area of each base station can be maximized, butso that the required traffic capacity will be attained.

A schematic structure of a cellular radio system accomplished with themicrocell technique in a town environment is as shown in FIG. 1, but aheavily built environment will set its own restrictions on the placingof base stations and radio wave propagation. The propagationpossibilities of a direct signal are restricted and the effect ofdiffractions and reflections on the propagation of radio waves isessential.

Vector maps to which the terrain and building information of the desiredarea are mapped are used as auxiliary means in network planning. Bymeans of a vector map, with the help of a computer, coverage areas andpropagation parameters are calculated for different locations of basestations and on the basis of these calculations, the most preferredpositions of base stations are selected. FIG. 2 illustrates by way ofexample a part of a vector map of a town area. The vector map describesthe buildings of the area to be examined with the desired accuracy,either specifically for each building or for each block on maps with alarger scale. The map of FIG. 2 comprises seven buildings 200 to 212.

In network planning there is an iterative process in which the basestation is placed in various locations and the coverage area of the basestation is determined for each location point. With the solution of theinvention, the calculation of the coverage area can be greatlyaccelerated by restricting calculation only to those areas to which aradio wave really propagates with the help of reflections anddiffraction. For each location point of the base station transmitter tobe examined, a polygon area is restricted from the calculation areadescribed by the vector map, radio waves being able to propagate to thepolygon area both directly and by means of diffraction and reflections.The strength of the emission of the transmitter is determined onlyinside the calculated polygon area.

The calculation of diffraction is examined first. In the method of theinvention, when the polygon area is determined, all the corners ofbuildings visible to the transmitter are examined, and if both sides ofthe corner to be examined are visible to the transmitter, the corner istaken as a corner included in the polygon as such, and if only one ofthe sides of the corner to be examined is visible to the transmitter,the intersection of the wall behind the corner and the corner is takeninto the polygon, as well. FIG. 3a shows a vector map describing a townarea. It is assumed that a corner 300 is visible to the base station.The diffraction polygon of the corner 300 is indicated in the map. Ifthe corner 300 is visible to the transmitter, the diffraction from thecorner 300 is calculated to the points inside the polygon shown in thefigure. The immediate environment of the corner 300 is shown magnifiedin FIG. 3b. The polygon to be determined is formed of visible corners302 to 348 of the corner 300 and the intersections behind them. Theintersections are indicated in the figure with small circles.

The corner points visible from the corner 300 are thus used as supplyinformation in calculating a polygon. The polygon can be determined bygoing through the corners 302 to 348 in order. If both sides of somecorner are visible to the corner, the corner is included in the polygonas such. In the figure the corner 314, for example, is such a corner. Ifonly one side is visible from some corner, the intersection of the wallbehind the corner and the corner are taken into the polygon. In thefigure this kind of a corner is a corner 308 where an intersection 350is included. In general, the polygon is formed in practical cases ofabout 20 to 150 corner points. These points of the polygon are stored inmemory. When the diffraction of some specific corner is calculated, thispolygon is taken from memory and it is checked which points are insidethis polygon and the diffraction is calculated to these points only.When using the method, it is not necessary to check if the corner inquestion is visible to the calculation point as it is known that all thepoints inside the polygon are visible to the corner.

The search for the points that are inside the polygon is quiteuncomplicated, and fast algorithms known to those skilled in the arthave been developed for the purpose.

The calculation of reflections is examined next. In the method of theinvention, when a polygon area is determined, a mirror image of thelocation of the transmitter with respect to the wall surface visible toeach transmitter is calculated, and that a sector restricted by the wallsurface to be examined from the location of the mirror image is taken asan area included in the polygon. The procedure is repeated recursively adesired number of times for the wall surfaces visible to the sectorrestricted by the wall surface to be examined from the location point ofthe mirror image.

FIG. 4 illustrates the calculation of multiple reflections with themirror image principle. The figure shows three buildings 400 to 404, atransmitter P and a receiver R. The source point P is mapped via assumedreflection surfaces to image points. After this it is examined if theroute in question exists, that is, if the ray can go to the destinationalong a route calculated in this way. In the figure reflection pointsP', P" and P'" are obtained by means of reflection surfaces via whichpoint the ray will advance to the receiver. By moving the receiver todifferent points, it is possible to detect when the route is not found.

Another known method used in the calculation of reflections is the raylaunching method. The movement of a ray from a transmitter is followed,and if it meets a wall, the reflected ray will be calculated and it willbe followed to the subsequent reflection point, and so on. Rays are sentfrom the transmitter at a distance of a specific angle difference. Thegenerally used angle difference is 0.1°, in which case there will be3,600 rays per point. At the reception point it is calculated which rayswill read from the vicinity of this point. If the ray moves closer than3 metres to the reception point, it can be assumed that the ray inquestion will attain the reception point. All the rays progressing viadifferent routes attaining the reception point are taken intoconsideration. After this the calculation of the next reception pointwill be started.

In the method of the invention, the two methods described above arecombined and a fast and accurate method for calculating reflections willbe obtained as a result. In the method, rays forming a sector in eachstarting and end point of the part of the wall visible from thetransmitter are calculated. By means of the rays, the rays reflectedfrom the wall surface are further calculated by means of a mirror imageof the transmitter. Then a new sector is formed where the image point ofthe transmitter is the starting and end point and the reflected rayssides. The sector is also restricted by the new parts of the wallencountered by the wave reflected from the wall, which parts arestarting points for new reflection sectors. The sector and the walls ofhouses restricting it form a polygon, that is, a reflection polygon toeach point of which can be calculated a reflection field via the wallsknown now. In this way it is possible to calculate a reflection field atthe same time for large areas by calculating just once the image pointswith known walls. The method will accelerate the calculation ofreflection fields of even large areas especially when more reflectionsare to be calculated. The developed method is also definitely accurateboth near to and far from the transmitter.

FIG. 5 illustrates the method of the invention described above. In thefigure, the transmitter is situated at point T. First, the sector 500,which forms the ray reflected from a wall surface W₁ of the building200, is calculated by means of a mirror image t' of the transmitter.Next by means of a mirror image t", a sector 502 reflected further froma wall surface W₂ of the building 202 is calculated, the sector beingindicated with oblique lines in the figure. By means of a mirror imaget'", a sector 504 reflected from a wall W₃ of the building 206 is alsocalculated, the sector being indicated with vertical lines in thefigure. The corresponding reflection calculations are carried out forall wall surfaces. In this example, triple reflection is described byway of example.

A reflection field can now be calculated for each point of thereflection polygon via the walls known now. As was stated earlier, thesearch for the points that are inside the polygon is quiteuncomplicated, and fast algorithms known to those skilled in the art aredeveloped for the purpose.

In the method of the invention, it is easy to combine diffraction andreflection calculation method described above in such a manner, forexample, that reflection is calculated first and diffraction after it orfirst diffraction and then reflection.

In the following, the structure of the apparatus accomplishing themethod of the invention will be examined by means of a block diagramshown in FIG. 6. The apparatus comprises means 600 for maintaining atleast a two-dimensional vector map describing the environment of thedesired base station. The means 600 are typically realized by means ofmemory circuits. The apparatus further comprises processor means 602 fordetermining the strength of the emission of the base station transmitterat various points in the environment. The apparatus of the inventionfurther comprises computation means 602 for determining a polygon areafrom the calculation area described by the vector map, radio waves beingable to propagate to the polygon area both directly and by means ofdiffraction and reflections, as in the above-described method, and means602 for estimating the strength of the emission of the transmitter ateach location point of the transmitter to be examined within thedetermined polygon area. Each location point of the transmitter undercalculation can be conveyed as an input 603 to the processor means. Themeans 602 can be preferably realized by means of a microprocessor or asimilar detached logic circuit, in which case the procedures of theinvention can preferably be realized by means of software.

As was described above, the vector map can comprise the outer edges ofthe buildings comprised in the environment of the base station. In thatcase the apparatus realizing the method of the invention comprises themeans 602 for calculating the diffraction field for each corner 300 ofthe buildings in such a manner that the calculation is carried out forthe corners 302 to 348 visible to said corner 300 and if both sides ofthe corner being examined are visible to the corner 300 to becalculated, the corner to be examined is taken as a corner included inthe polygon as such, and if only one of the sides of the corner to beexamined is visible to the corner 300 to be calculated, the intersectionof the wall behind the corner to be examined and the corner is alsotaken into the polygon.

By means of one preferred embodiment, the processor means 602 of theapparatus realizing the method of the invention calculate polygon pointsfor each corner point in advance. The memory means 604 store the pointsfor calculation. The processor means 602 read the points from a memorywhen the coverage area is calculated for different location points ofthe transmitter.

Although the invention is explained above with reference to the exampleof the accompanying drawings, it is evident that the invention is notrestricted thereto, but it can be modified in various ways within theinventive idea disclosed in the appended claims.

We claim:
 1. A method for determining path attenuation of radio waves ina radio system,in which method at least a two-dimensional vector mapdescribing the environment of a base station is used for determining thecoverage area of the base station of the system, and in which thestrength of the emission of a transmitter is determined at variouspoints in the environment, characterized by calculating from the basestations environment described by the vector map a polygon area to whichradio waves can propagate both directly and by means of diffraction andreflections, by estimating the strength of the emission of thetransmitter at each location point of the transmitter to be examined inthe polygon area determined by calculation, and by determining points ofthe polygon area in advance, storing the determined points in a memory,and reading the stored points from the memory for calculating thepolygon area for different location points of the transmitter.
 2. Amethod according to claim 1, characterized in that the vector mapdescribes the outer edges of the buildings comprised in the environmentof the base station, and thatwhen determining the polygon area, thecorners of the buildings visible to the transmitter are examined, andwhen calculating a diffraction field for a selected corner (300), othercorners (302-348) visible to the selected corner (33) are examined, andif both sides of one of the other corners to be examined are visible tothe selected corner (300) to be calculated, the one of the other cornersto be examined is taken as a corner included in the polygon as such, andif one of the sides of the one of the other corners to be examined isvisible to the selected corner to be calculated, the intersection of thewall behind the one of the other corners and the one of the othercorners is also taken into the polygon.
 3. A method according to claim2, characterized in that the points of the polygon area that aredetermined in advance are points representing the selected corner andthe other corners.
 4. A method according to claim 1, characterized inthat the vector map describes the outer edges of the buildings comprisedin the environment of the base station, and thatwhen determining thepolygon area, a mirror image (t') is calculated for the location of thetransmitter with respect to each wall surface (W₁) visible from thetransmitter (T), and that a sector (500) restricted by the wall surfaceto be examined from the location of said mirror image is taken as anarea included in the polygon.
 5. A method according to claim 4,characterized in that the procedure is repeated recursively a desirednumber of times for wall surfaces (W2, W3) visible to the sectorrestricted by the wall surface to be examined from the location of saidmirror image (t', t").
 6. An apparatus for determining path attenuationof radio waves in a radio system comprising at least one basestation,and which apparatus comprises means (600) for maintaining atleast a two-dimensional vector map describing the environment of adesired base station, and means (602) for determining the strength ofthe emission of a transmitter at various points in the environment,characterized in that the apparatus further comprisesmeans (602) forcalculating from the base station environment described by the vectormap a polygon area to which radio waves can propagate both directly andby means of diffraction and reflections, means (602) for estimating thestrength of the emission of the transmitter at each location point ofthe transmitter to be examined inside the determined polygon area, andmeans for determining points of the polygon area in advance, means forstoring the determined points in a memory and means for reading thestored points from the memory for calculation of the polygon area insaid means for calculating for different location points of thetransmitter.
 7. An apparatus according to claim 6, characterized in thatthe apparatus comprises means (602) for maintaining a map of the outeredges of the buildings comprised in the environment of the basestations, andmeans (602) for calculating a diffraction field for aselected corner (300) of the buildings in such a manner that thecalculation is carried out for other corners (302-348) visible to theselected corner (300) and if both sides of the one of the other cornersbeing examined are visible to the selected corner (300) to becalculated, the one of the other corners to be examined is taken as acorner included in the polygon as such, and if only one of the sides ofone of the other corners to be examined is visible to the selectedcorner (300) to be calculated, the intersection of the wall behind theone of the other corners to be examined and the one of the other cornersis taken into the polygon.
 8. An apparatus according to claim 6,characterized in that the points of the polygon area that are determinedin advance are points representing the selected corner and the othercorners.